The properties of dental filling composites depend on the structure of the organic matrix but also on the properties of the fillers used. Particle size, size distribution, particle shape, type of particle surface, chemical composition, the nature of any surface modification and optical properties of the fillers and also the total fillers content have a major influence on the overall properties of the composite (cf J. F. Roulet, Degradation of Dental Polymers, Karger, Basel 1987, page 10).
A high strength of the dental filling composites as well as a low polymerization shrinkage can be achieved above all by a high degree of filling. The optimum degree of filling is also determined by the necessary consistency of the composite pastes which are used in practice. These must be such that an optimum introduction of the materials into the tooth cavity as well as an optimum processing is possible. It is known that, as a result of the addition of fillers to the matrix monomer mixture, viscosity increases with the filler content, the thickening effect e.g. of the highly dispersed silicic acid, which according to DE-C-24 03 211 is used as filler in dental compositions, growing as the primary particle size decreases and the BET surface increases. Currently used microfiller composites are consequently characterized by a degree of filling with inorganic fillers of ca. 50 wt. % and hybrid composites by a degree of filling of ca. 80 wt. %.
By contrast, lower viscosities are necessary in the case of composite-based fixing cements compared with the corresponding permanent filling composites, so that only a lower degree of filling can be achieved. A highly fluid consistency is generally necessary for dental adhesives, for which reason the addition of fillers must be completely dispensed with as a rule in these.
The use of products of sol-gel processes as a constituent of dental materials is known. Thus, dental materials which contain heterosiloxanes as filler of small particle size are described in DE-C-39 13 252, EP-B-394 794 and EP-B-523 545. The heterosiloxanes used are prepared by cocondensation of suitable silanes with metal alkoxides as statistical copolycondensates, block copolycondensates or so-called mixed copolycondensates. It is necessary that, after carrying out the sol-gel process, the solids produced are separated, washed, dried, ground while being kept at a certain temperature and in some cases also functionalized by surface silanization. These process steps influence the properties of the finally obtained filler in such a way that the latter is present merely in agglomerated form.
Analogous sol-gel fillers based on heteropolysiloxanes can be used for dental filling materials according to EP-B-381 961.
Also known, from DE-A-41 33 494, are dental materials based on polymerizable polysiloxanes which are likewise obtained by the sol-gel process from hydrolytically condensable silanes. The resins obtained after condensation are highly viscous and can hardly be filled further.
Compositions based on organically modified silicic acid-poly-condensates, which can be used for coating teeth and denture parts, are known from WO 92/16183. It is necessary for the obtained inorganic/organic precondensates to be diluted with a solvent in order to control the viscosity of the compositions.
GB-A-2 257 438 discloses products of the sol-gel process for the glass-like coating of teeth.
Finally, it is also known that the preparation of SiO.sub.2 sols by hydrolysis and condensation of suitable precursors, e.g. condensable silanes, is possible. The sol particle formation depends among other things on the nature of the precursors, the nature of the reaction medium, the pH value, the catalyst or the amount of water used (cf C. J. Brinker, G. W. Scherer, Sol-Gel-Science, Academic Press, Boston 1990, pages 99 et seq. and 617 et seq.).
Silica sols are aqueous solutions of colloidal, amorphous SiO.sub.2 which as a rule contain 10 to 50 wt. % SiO.sub.2 particles having a diameter of 5 to 150 nm (cf Ullmann's Encyklopadie der technischen Chemie, 4th edition, Volume 21, Verlag Chemie, Weinheim 1982, pages 456 et seq.). It is possible to silanize the particles of these aqueous silica sols e.g. with 3-(methacryloyloxy)-propyltrimethoxysilane and carry out a solvent replacement, e.g. with isopropanol or monomer (cf L. N. Lewis et al., Polym. Mat. Sci. Techn., Proceed. Amer. Chem. Soc., 72 (1995), page 583).